The present invention relates generally to methods for measuring properties in rock, and more particularly toward a highly sensitive, in-situ method of measuring material properties (mechanical properties) of rock surrounding a borehole.
For various scientific and economic reasons, it may be necessary to determine mechanical properties, such as elastic modulus and Poisson's ratio, in the material surrounding a borehole in rock. Mechanical properties may be of interest in scientific geologic studies. Knowledge of mechanical properties may be required in the engineering of measures to facilitate fluid flow in the rock body. Knowledge of the in-situ state of stress (the natural state of stress in the rock body) can be of critical importance in the design of subterranean structures; when the in-situ state of stress is to be determined, mechanical properties must be known in order to interpret accurately additional measured data from which the state of stress may be inferred.
The term "stress" as used herein is defined as a force acting across a unit area in rock in resisting separation, compacting or sliding that tends to be induced by external forces. "Strain" is a change in length of an object in some direction per unit undistorted length in the same or different direction. "Elastic modulus" or "modulus of elasticity" is the ratio of an increment of stress to an increment of strain. "Poisson's ratio" is the ratio of a transverse contracting (expanding) strain to the elongation (contraction) strain when a rod is stretched (compressed) by forces which are applied at its ends and which are parallel to the rod's axis. "Torsional stiffness" is the ratio of a steady torsional force acting on a rock plug to the resulting angular displacement. See generally, Dictionary of Scientific and Technical Terms, McGraw-Hill, 1974.
Prior art devices that are theoretically capable of remote measurements of material properties in a borehole demonstrate poor accuracy in laboratory and field tests. Typically, a relatively small piston is pressed into the wall of a borehole (see, e.g., Edmond et al U.S. Pat. No. 4,030,345) to develop a displacement signal of inherently low sensitivity. Measuring displacement across the borehole or strain in the borehole wall along an axis orthogonal to the load axis is similarly insensitive and subject to difficulties in obtaining credible data from displacement or strain transducers.
Other problems associated with prior art in-situ or remote measurement of material properties of borehole wall rock include poor repeatability in successive testing of the same sample, calibration problems due to irregularities at the piston-wall interface and poor localization of the point of measurement.